1. Field of the Invention
This invention relates to a broadcast or multicast service in a telecommunications system. More explicitly, but not exclusively, the invention relates to the realisation of a Multicast Broadcast services in a radio access network (RAN) such as in the Universal Mobile Telecommunications Service (UMTS) radio access network. UMTS concerns a third generation radio network using wideband code division multiple access (W-CDMA) technology.
2. Description of the Related Art
A cellular communications system includes mobile user equipment (UEs), a radio access network (RAN) and one or more core networks (CNs), as illustrated in FIG. 1 for the UMTS case. A detailed overview over the architecture of a cellular telecommunications system of the third generation may be found in the 3GPP specification “UTRAN Overall Description” 3GPP TS25.401 and related specifications. Communication between the UEs and the UTRAN is provided via the Uu interface (Uu), whereas the communication between the UTRAN and the core networks is done via the Iu interface (Iu).
A radio access network includes base stations and radio network controllers or base station controllers (RNC/BSC). The base stations handle the actual communication across the radio interface, covering a specific geographical area also referred to as a cell. The radio network controllers control the base stations connected to it, but in addition perform other functionality like for example the allocation of radio resources and the control of local mobility. An RNC connects to one or more core networks via the Iu interface, to a number of base stations (node B's for the case of UTRAN) via the Iub interface and possibly to one or more other RNCs via the Iur interface. The core network includes a serving GPRS (General Packet Radio Service) support node (SGSN) and a broadcast/multicast-service centre (BM-SC). The BM-SC controls the distribution of the data to be transmitted via the MBMS service.
Communications Networks of the third generation (3G) such as the UMTS network provide Multimedia Broadcast Multicast Services (MBMS). MBMS is a point-to-multipoint service in which multimedia data such as audio, images or video data is transmitted from a single source entity to multiple recipients by using an unidirectional bearer service. The MBMS bearer service offers both a broadcast mode and a multicast mode. In the broadcast mode, the data are broadcasted to all users. In contrast, a user needs to subscribe to a particular MBMS service or a group of MBMS services with a service provider in order to receive multicast services. The operator may then announce the service or use a service discovery mechanism to inform users about the range of MBMS services available. If the user is interested in a particular MBMS service, the user joins the service, i.e. the user activates the MBMS multicast service. In this way the user becomes a member of a particular multicast group and indicates to the network that he or she wants to receive the MBMS data of a particular MBMS service.
Transmitting the same data to multiple recipients allows network resources to be shared. In this way the MBMS architecture is designed to enable an efficient usage of radio-network and core-network resources.
In order to initiate a MBMS session, the CN sends a session start command to the RNC. The Session Start command is used to indicate that the core network is ready to send data relating to a particular MBMS service. The Session Start command triggers the establishment of a bearer resource for MBMS data transfer. It is noted that the Session Start occurs independently of activation of the service by the user. This means that a user may activate a particular service either before or after a Session Start.
After receiving the Session Start command, the RNC send MBMS notifications to the UE in order to inform the UEs about forthcoming or even ongoing MBMS multicast data transfers. The RNC manages the use of the radio resources and decides whether the MBMS data will be transmitted using point to multipoint or point-to-point transfer mode on the radio interface. If there are sufficient UEs in a cell, the point-to-multipoint transfer mode is most efficient. If however the number of users in a cell is low, the point-to-point transfer mode may be most efficient. To decide which transfer mode to use, the RNC may perform a counting operation. Subsequently multimedia data relating to a particular MBMS service are transmitted from the CN via the RNC to the UEs during the data transfer phase.
When the BM-SC determines that there will be no more data to send, the CN sends a Session Stop command to the RNC and the bearer resources are released.
If a user is no longer interested in a particular MBMS service, the user deactivates the service. Accordingly, the user leaves the multicast group if he or she does no longer wants to receive Multicast mode data of a specific MBMS bearer service.
It is noted that the phase subscription, joining and leaving are performed individually per user. The other phases, such as the notification and the data transfer, are performed for a particular service, i.e. for all users interested in the related service.
As of 3GPP RAN2 meeting number 39 (17-21 Nov. 2003), the situation regarding how MBMS will be handled on the Uu interface has again become clearer. The current status of MBMS realisation in RAN2 is described in the 3GPP specification “Introduction of Multimedia Broadcast Multicast Service (MBMS) in the Radio Access Network (RAN)” TS 25.346. v2.4.0.